Ontario in total has nearly 40% of the population, and most of that is pretty far south relative to much of the rest of the country.
As I was reading it, I looked at the map, and concluded that a bunch of the population was likely in the Toronto area which juts well below the mean longitude of Canada's southern border.
For example, in BC the population centers are on the southern border (thanks to a) the Fraser river, and b) politics). But in Alberta they really are not. Nor Saskatchewan. Manitoba sort of is, by geography. So if you were to do this analysis on western Canada, it wouldn't be nearly as clear cut.
Most of "the south" of Canada doesn't have much similarity with south western Ontario, for that matter.